Sinn Fein president Gerry Adams has initiated defamation proceedings against the BBC over a Spotlight investigation.
The case is being taken in the High Court in Dublin, it has been learned.
The programme investigated the killing of Denis Donaldson (55), a former Sinn Fein official who was exposed as a British spy, having worked for M15 for two decades.
The Belfast man was shot dead at his isolated cottage near Glenties, Co Donegal in April 2006.
The Spotlight programme was broadcast in September last year.
The man who made claims about Mr Adams said he was a paid state agent in the IRA.
Mr Adams indicated at the time that he intended to sue the BBC.
The proceedings were initiated on Tuesday and Mr Adams is being represented by high-profile defamation lawyer Paul Tweed, of Johnsons Solicitors.
Sinn Fein referred a request for comment to Mr Tweed’s office last night. However, Mr Tweed was unavailable.
At the time Mr Adams vehemently denied the claims made in the broadcast.
He attributed them to “elements within the British system” of trying to undermine him and his party.
“We have to look at what’s behind this agenda,” he said.
“The person who made this allegation — an anonymous, unnamed, self-professed agent of the British state. So whose agenda is that serving? This is an attempt to rewrite history.
“There are elements within the British system who will never be reconciled with the fact that we have got a peace process and that Sinn Fein are in the leadership of that process, along with others.”
The BBC has previously stated it stands by its journalism, but no comment was forthcoming from the broadcaster last night.
The dissident republican group the Real IRA claimed responsibility for Mr Donaldson’s murder in 2008.
No one has ever been prosecuted in connection with his death.